Electronic apparatus is often powered from a mains electricity supply. Such electronic apparatus comprises power conversion apparatus which is operative to convert a high voltage alternating current (AC) supply to a low voltage direct current (DC) supply. Power conversion apparatus can be considered to belong to one of two different classes depending on whether or not the power conversion apparatus comprises a transformer. Transformers provide for isolation and relatively loss free voltage level conversion, amongst other things, but on the other hand they are typically large, heavy and expensive. Transformerless power conversion apparatus are comparatively inexpensive and small but are limited to low power applications and typically to less than 40 mW of direct current output power. At present transformerless power conversion apparatus are used mainly in AC powered applications which are operative on the mains side, such as in electricity meters, residual current detectors, home control and monitoring, PIR controlled exterior lights and fire alarms.
Transformerless power conversion apparatus can be considered to belong to one of two classes, namely resistive transformerless power conversion apparatus and capacitive transformerless power conversion apparatus. Resistive transformerless power conversion apparatus are normally wasteful of power and therefore limited to few low power applications. Capacitive transformerless power conversion apparatus are less wasteful of power than resistive transformerless power conversion apparatus but yet may waste more than ten to twenty times the output power.
A simple representation of capacitive transformerless power conversion apparatus 10 is shown in FIG. 1. The power conversion apparatus 10 comprises a live connection 12 and a neutral connection 14 which are connected respectively to the live and neutral conductors of a mains electricity supply, such as a 240 V AC electricity supply. A first capacitor 16 and a resistor 18 are in series with the live connection. The first capacitor 16 is an X type capacitor which is designed to provide for safety at mains voltage levels. The resistor 18 is present to limit the inrush current to the first capacitor 16 if the power conversion apparatus is connected to the mains at a point in the mains cycle other than the zero crossing or in the event of a mains surge. The power conversion apparatus 10 further comprises a diode 20 in series with the first capacitor 16 and the resistor 18 which is oriented such that its anode is electrically connected to the resistor 18. The power conversion apparatus 10 also comprises a Zener diode 22 and a holding capacitor 24. The cathode of the Zener diode 22 is electrically connected between the resistor 18 and the diode 20 and the anode of the Zener diode 22 is electrically connected to the neutral connection 14. The holding capacitor 24 is electrically connected between the cathode of the diode 20 and the neutral connection. The power conversion apparatus 10 yet further comprises a positive voltage output connection 26 at the diode 20 side of the holding capacitor 24 and a low voltage output connection 28 at the neutral side of the holding capacitor 24. The positive and low voltage output connections 26, 28 constitute the output from the power conversion apparatus 10. The breakdown voltage of the Zener diode 22 minus the voltage drop across the diode 20 determines the voltage across the holding capacitor 24 and hence the output voltage from the power conversion apparatus 10.
The present inventors have appreciated that the Zener diode 22 in the power conversion apparatus of FIG. 1 provides for consumption of the same power irrespective of the power output from the power conversion apparatus. This is because the consumed power comprises power shunted by the Zener that is not used by the load. The greatest power conversion efficiency is obtained when the power conversion apparatus is operating under full load conditions with there being a progressive reduction in power conversion efficiency as the load decreases. The present inventors have also appreciated that the power conversion efficiency of the power conversion apparatus is compromised by the power dissipated by the inrush current limiting resistor 18 and the voltage dropped across the Zener diode 22 irrespective of load conditions. The power dissipated by the resistor 18 is a function of the reactance of the first capacitor 16 and the resistance of the resistor 18.
The present invention has been devised in the light of the above mentioned appreciations. It is therefore an object for the present invention to provide improved power conversion apparatus which is configured to receive a high voltage alternating current signal at an input and to provide in dependence thereon a low voltage direct current signal from an output stage. It is a further object for the present invention to provide an improved power conversion arrangement comprising plural such power conversion apparatus.